Tolerance of Topological Surface States towards Magnetic Moments: Fe on ${\mathrm{Bi}}_2{\mathrm{Se}}_3$

We study the effect of Fe impurities deposited on the surface of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ by means of core-level and angle-resolved photoelectron spectroscopy. The topological surface state reveals surface electron doping when the Fe is deposited at room temperature and hole doping with increased linearity when deposited at low temperature ($\sim 8\mathrm{K}$). We show that in both cases the surface state remains intact and gapless, in contradiction to current belief. Our results suggest that the surface state can very well exist at functional interfaces with ferromagnets in future devices.

Figure 1

Behavior of topological surface states upon deposition of Fe. Angle-resolved photoemission data at room temperature for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The red dashed line follows the $n$-type (electron) surface-doping effect on the Dirac point ($E_{\mathrm{D}}$). The Dirac point is clearly observed, and saturation of the surface-doping effect is seen at 0.3 ML Fe. The photon energy is 18 eV.

Figure 2

Comparison of room-temperature and low-temperature deposition at the same measurement temperature of 8 K. Extra $\mathsf{M}$-shaped bands in the valence-band range are seen to coexist with the Dirac point. The photon energy is 18 eV in (a),(b) and 21.5 eV in (c),(d).

Figure 3

Fe deposition at low temperature yields a shift of the Dirac point to lower binding energy (hole doping) and enhanced linearity in two subsequent deposition steps. Cleavage, Fe deposition, and measurement were done at 8 K. No gap appears in the topological surface state. The photon energy is 18 eV in (a) and 21.5 eV in (b),(c).

Figure 4

Core-level spectroscopy distinguishes the different deposition methods. (a),(c) The $\mathrm{Se}3d$ core levels cover the $\mathrm{Fe}3p$ emission. $\mathrm{Se}3d$ shifts to higher binding energy only for low-temperature (8 K) deposition of the Fe. (b),(d) $\mathrm{Bi}5d$ develops shoulders with the core-level shifts having different values for room-temperature and low-temperature preparation. The photon energy is 125 eV, and temperatures were the same during deposition and measurement.